Conventional integrated circuit packages are typically made up of a silicon chip packaged in a ceramic or plastic housing. Such packages further typically include a plurality of integrally attached leads extending from the perimeter of the housing. The leads form the physical attachments and conductive paths by which the chip is connected to some higher level assembly, usually a printed circuit board.
Chip-on-tape technology, which is also known as "tape automated bonding", or "TAB", involves a number of modifications to this conventional approach. More particularly, chip-on-tape technology uses IC's which are manufactured without integrally attached leads extending from their perimeter. Rather, the IC package is formed with tiny, contact pads, usually positioned along the perimeter of the underside of the package. Before such a package is integrated into a higher assembly, such as a printed circuit board, each contact pad is bonded to a corresponding conductive lead on a tape lead frame which is configured for having the chip mounted thereon. Once the chip is bonded to the tape, the free ends of the conductive leads on the tape can be bonded to a printed circuit board. Accordingly, the conductive leads on the tape serve as the functional substitutes for the fixed leads integrally formed with the conventionally designed package.
The tape that is used in a chip-on-tape application is often a two sided, flexible connector. On one side is a dielectric film, the appearance of which somewhat resembles the film that goes in an ordinary camera. On the other side, and integrally attached to the film, are the conductive leads which are precisely configured so that one end of each lead mates with one contact pad on an IC, and the other end mates with one contact pad on a printed circuit board. Therefore, once the chip is mounted onto the tape, the chip and tape package must be appropriately placed on the printed circuit board so that each lead can be bonded with its corresponding pad.
There are multiple methods by which an IC package can be surface mounted on a printed circuit board, such as mass reflow, hot bar, single point bonding, and laser bonding. When surface mounting using the laser bonding technique, the IC package, either conventional or TAB, is first positioned on the printed circuit board so that each of the leads is aligned for bonding with a flat surface of its corresponding contact pad on the printed circuit board. After so mounting the package, the lead/pad joint is heated by means of a laser to cause the reflow of a solder composition which subsequently will cool and harden to form a good physical and electrical contact.
An impediment to this type of bonding process is that when the package is surface mounted on the board all of the leads may not make flat, intimate contact with their corresponding pads. Flat, intimate contact is necessary to assure that the leads and pads are physically joined during the laser bonding process. Additionally, such contact enables the laser energy absorbed by the lead to be conducted to the pad resulting in the reflow necessary for bond formation.
In practice, there are a number of reasons why a given lead on a package fails to set flatly against its corresponding contact pad, a condition which will hereinafter be referred to as "non coplanarity" between the lead and pad. For example, during the lead forming process, which is the process in which a straight lead is bent, typically in the shape of a gull wing, the lead formation may not be perfect resulting in non coplanarity. In addition, in some surface mount applications, each pad is covered with a solder dome. If, however, the solder domes are not uniform from pad to pad, this too can cause non coplanarity of some leads and their corresponding pads. Other causes of non coplanarity between leads and pads could be the unevenness of the housing of the package, warpage of the printed circuit board, or component distortion.
One approach to keeping the leads and pads in intimate contact during the laser bonding process is shown in U.S. Pat. No. 4,978,835, issued to Luijtjes, et al. on Dec. 18, 1990. In Luijtjes the first and second contacts (ie. the leads and the pads) are bonded by first aligning the two contacts, placing a diaphragm against the first contact, and then applying a differential pressure to the diaphragm to pull the first contact in the direction of the second contact. Thus, the two contact elements are kept in intimate contact through the pressure applied by the diaphragm, which is made of a glass plate or transparent membrane. According to Luijtjes, the bonding is accomplished by directing a laser beam at the diaphragm overlaying the contacts to cause reflow.
A drawback to the approach shown in Luijtjes, if applied in a bonding process which uses solder flux, is that the diaphragm provides a surface on which the condensates resulting from the bonding process may accumulate. When bonding with a flux, often the flux may vaporize after irradiation by the laser, and the resulting condensates will deposit on the diaphragm. Consequently, during subsequent use the deposits may cause some laser beam attenuation due to refraction, reflection, and absorption. Such beam attenuation introduces additional process variables into the bonding operation which may be undesirable because those variables may impair the reliable formation of a bond.
Another drawback to the approach shown in Luijtjes is that it requires the matching of the optical properties of the diaphragm with the laser beam characteristics, such as wavelength and power density. Without such matching, the diaphragm may be unable to transmit the proper amount of beam energy to cause solder reflow and may be damaged due to its direct optical absorption of the incoming laser beam.
What is needed is a hold-down fixture which maintains the leads in intimate contact with the pads during bonding, and avoids the drawbacks of the prior art.